Q&A Forum
I recently acquired the shop and dyno that I originally set up for the previous owner. I opted to configure a brand-new PC and install the latest YourDyno Una Vision software. The dyno's inertia parameters are dialed in correctly; I get very clean, repeatable data in inertia-only mode. On the test truck I’m using right now, inertia pulls consistently show about 270 wheel torque.
The issue is that when I switch to a brake-controlled power sweep, reported torque drops to roughly 140 wheel torque, less than half of the inertia value. Everything else appears to be functioning normally: the brake responds to commands, RPM pickup is clean, and the load cell zeroes properly.
I have attached screenshots of the power sweep configuration and load cell setup screens.
Any ideas of what I'm doing wrong?
Here is the sequence of calibrating a new setup:
- Ensure the number of pulses per revolution is correct in the RPM setup
- Calibrate the load cell(s). The procedure is described in the UnaVision manual
- Dial in the brake settings so a Power sweep or Load control run is smooth
- Measure the Moment of Inertia. There are several ways to do it. I like to do one inertia only run and one slow brake controlled run. It is essential here that the braked run is much slower than the inertia run. Adjust the MOI such that the inertia run and the braked run overlap as much as possible. For the inertia measurement, choose a car that
- Does not change gear during the run
- Has a very tight torque converter (best with a manual or double clutch car)
- Does not heat soak or adjusts the turbo pressure or similar based on the load or anything else that would make the engine perform differently in the two runs. Also make sure the air in the shop is clean for both runs
- Select Power correction % and or Retardation measurement as you want
- I always recommend doing the retardation measurement, as it will include the unmeasured losses in the dyno, as well as the unloaded losses in the tires and drive train
- In addition to the retardation measurement, you need to add some Power correction if you want engine power. For an efficient drive train, 5% is typical
Note that when you search for the right MOI, you can change the MOI and click the re-run button on the two runs you are comparing. No need to do the physical runs again
Okay, so load cell calibration is definitely my issue. MOI is actually recorded on a tag on the side of my dyno, so that should be accurate. I can confirm later, but I've always used that number and it's been at least pretty close.
I had previously calibrated the load cell the old way, as demonstrated in your 2016 video: removing the load cell and putting a weight directly on it. I now see that the software requires a different calibration process.
I cannot mount a calibration arm to the load cell in place on the dyno, as it lays horizontally, but I'm able to remove it and rig it up in a vertical position. I followed the instructions as they are laid out in the software manual. Even using a 58" calibration arm and a 25lb weight plate, the raw load cell reading only displays about 6% of max. Something seems wrong here, as the amount of force being placed on the load cell is extensive!
In this post I'm attaching an image of how the load cell is mounted on my dyno.
For rigging up a calibration solution, I went ahead and flipped the load cell mounting bracket and bolted the load cell directly to it. I bolted my 58" calibration arm to the top of the cell. At this point I clicked Zero Calibrate. I entered 58.0" into my "Distance to calibration weight" field, and 25.00 in my "Calibration weight" field. I then added my 25lb weight and clicked Load Calibrate. This is when I saw the Raw loadcell reading was displaying 6% of max. Then I completely removed my weight and calibration arm and pressed Zero calibrate button again, and then clicked Save calibration.
I strapped down my Corvette with a manual transmission to ensure repeatability. I did 2 back-to-back pulls. Inertia only displayed 365 tq, which is about where it should be. I then did a power sweep and got over 2000 tq.
Image attached shows my temporary load cell mounting and calibration arm setup. I understand that it will be better to have the weight hanging from the arm rather than sitting on it, but I'm just trying to get into the general ball park right now, which I'm completely missing the mark on.
Hi!
Looking for the last image... Seems that the lorque arm is mounted not as it should.
The load cell could handle an axial force only - it means that you must avoid any forces except tension/compression.
Your setup "bends" the load cell and the readings of YourDyno are wrong, because you see some values based on the physical deformation of the load cell.
As @doctor-542 mentions, the last picture does not show a correct calibration.
You can think of load cell calibration as Torque calibration. You apply a certain torque (weight x arm) to the brake and tell YourDyno that this is the torque it should read. It does not matter how the load cell is mounted.
If it is impossible to apply a torque to the brake (that would be an unusual case) then take the load cell off and place it on a table. Put the distance from the middle of the brake to the load cell as the distance to calibration weight and the calibration weight in lbs as the weight. Then do a calibration as was done in the old 2016 video.